1/* Copyright (C) 2002-2017 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3 Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <http://www.gnu.org/licenses/>. */
18
19#include <errno.h>
20#include <stdlib.h>
21#include <fork.h>
22#include <atomic.h>
23#include <futex-internal.h>
24
25
26void
27__unregister_atfork (void *dso_handle)
28{
29 /* Check whether there is any entry in the list which we have to
30 remove. It is likely that this is not the case so don't bother
31 getting the lock.
32
33 We do not worry about other threads adding entries for this DSO
34 right this moment. If this happens this is a race and we can do
35 whatever we please. The program will crash anyway seen. */
36 struct fork_handler *runp = __fork_handlers;
37 struct fork_handler *lastp = NULL;
38
39 while (runp != NULL)
40 if (runp->dso_handle == dso_handle)
41 break;
42 else
43 {
44 lastp = runp;
45 runp = runp->next;
46 }
47
48 if (runp == NULL)
49 /* Nothing to do. */
50 return;
51
52 /* Get the lock to not conflict with additions or deletions. Note
53 that there couldn't have been another thread deleting something.
54 The __unregister_atfork function is only called from the
55 dlclose() code which itself serializes the operations. */
56 lll_lock (__fork_lock, LLL_PRIVATE);
57
58 /* We have to create a new list with all the entries we don't remove. */
59 struct deleted_handler
60 {
61 struct fork_handler *handler;
62 struct deleted_handler *next;
63 } *deleted = NULL;
64
65 /* Remove the entries for the DSO which is unloaded from the list.
66 It's a single linked list so readers are. */
67 do
68 {
69 again:
70 if (runp->dso_handle == dso_handle)
71 {
72 if (lastp == NULL)
73 {
74 /* We have to use an atomic operation here because
75 __linkin_atfork also uses one. */
76 if (catomic_compare_and_exchange_bool_acq (&__fork_handlers,
77 runp->next, runp)
78 != 0)
79 {
80 runp = __fork_handlers;
81 goto again;
82 }
83 }
84 else
85 lastp->next = runp->next;
86
87 /* We cannot overwrite the ->next element now. Put the deleted
88 entries in a separate list. */
89 struct deleted_handler *newp = alloca (sizeof (*newp));
90 newp->handler = runp;
91 newp->next = deleted;
92 deleted = newp;
93 }
94 else
95 lastp = runp;
96
97 runp = runp->next;
98 }
99 while (runp != NULL);
100
101 /* Release the lock. */
102 lll_unlock (__fork_lock, LLL_PRIVATE);
103
104 /* Walk the list of all entries which have to be deleted. */
105 while (deleted != NULL)
106 {
107 /* We need to be informed by possible current users. */
108 deleted->handler->need_signal = 1;
109 /* Make sure this gets written out first. */
110 atomic_write_barrier ();
111
112 /* Decrement the reference counter. If it does not reach zero
113 wait for the last user. */
114 atomic_decrement (&deleted->handler->refcntr);
115 unsigned int val;
116 while ((val = deleted->handler->refcntr) != 0)
117 futex_wait_simple (&deleted->handler->refcntr, val, FUTEX_PRIVATE);
118
119 deleted = deleted->next;
120 }
121}
122